Sign up to receive free email alerts when patent applications with chosen keywords are publishedSIGN UP

Abstract:

Methods and apparatus provide for forming a semiconductor-on-insulator
(SOI) structure, including subjecting a implantation surface of a donor
semiconductor wafer to an ion implantation step to create a weakened
slice in cross-section defining an exfoliation layer of the donor
semiconductor wafer; and subjecting the donor semiconductor wafer to a
spatial variation step, either before, during or after the ion
implantation step, such that at least one parameter of the weakened slice
varies spatially across the weakened slice in at least one of X- and Y-
axial directions.

Claims:

1. A method of forming a semiconductor-on-insulator (SOI) structure,
comprising: subjecting a implantation surface of a donor semiconductor
wafer to an ion implantation step to create a weakened slice in
cross-section defining an exfoliation layer of the donor semiconductor
wafer, where the donor semiconductor wafer has a width, a depth and a
height, the width and depth defining X- and Y- axial directions, and the
height defining a longitudinal axis; and subjecting the donor
semiconductor wafer to a spatial variation step, either before, during or
after the ion implantation step, such that a depth of the weakened slice
from the implantation surface resulting from the ion implantation step
varies spatially across the weakened slice in at least one of the X- and
Y- axial directions.

2. The method of claim 1, wherein: the donor semiconductor wafer is
rectangular; and the spatial variation step includes spatially varying
the depth of the weakened slice such that a substantially high depth
exists at each of at least two edges of the weakened slice of the donor
semiconductor wafer and comparatively lower depths exist at successively
further distances from the at least two edges toward a center of the
weakened slice.

3. The method of claim 2, wherein the spatial variation step includes
spatially varying the depth of the weakened slice such that a
substantially high depth exists at all edges of the weakened slice and
comparatively lower depths exist at successively further distances toward
the center of the weakened slice.

4. The method of claim 1, further comprising elevating the donor
semiconductor wafer to a temperature sufficient to initiate separation at
the weakened slice from a point, edge, and/or region of lowest depth of
the weakened slice.

5. The method of claim 4, further comprising elevating the donor
semiconductor wafer to further temperatures sufficient to continue
separation substantially along the weakened slice directionally as a
function of the varying depth, from lowest depth to highest depth.

6. A method of forming a semiconductor-on-insulator (SOI) structure,
comprising: subjecting a implantation surface of the donor semiconductor
wafer to an ion implantation step to create a weakened slice in
cross-section defining an exfoliation layer of the donor semiconductor
wafer; boring a blind hole through the implantation surface at least to
the weakened slice; and elevating the donor semiconductor wafer to a
temperature sufficient to initiate separation at the blind hole.

7. The method of claim 6, further comprising: boring an array of blind
holes through the implantation surface at least to the weakened slice to
create a non-uniform spatial distribution; and elevating the donor
semiconductor wafer to further temperatures sufficient to continue
separation substantially along the weakened slice directionally as a
function of the distribution of the array of blind holes.

Description:

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application is a divisional application of U.S. patent
application Ser. No. 12/779,606, filed on May 13, 2010 and entitled
"METHODS AND APPARATUS FOR PRODUCING SEMICONDUCTOR ON INSULATOR
STRUCTURES USING DIRECTED EXFOLIATION," which, in turn, is a divisional
application of U.S. patent application Ser. No. 12/290,362, filed on Oct.
30, 2008 and entitled "METHODS AND APPARATUS FOR PRODUCING SEMICONDUCTOR
ON INSULATOR STRUCTURES USING DIRECTED EXFOLIATION," the content of both
of which are incorporated herein by reference in their entirety.

TECHNICAL FIELD

[0002] The present invention relates to the manufacture of
semiconductor-on-insulator (SOI) structures, such as those of
non-circular cross section and/or those of relatively large cross
sectional area.

[0004] Various ways of obtaining SOI structures include epitaxial growth
of silicon (Si) on lattice matched substrates, and bonding a single
crystal silicon wafer to another silicon wafer. Further methods include
ion-implantation techniques in which either hydrogen or oxygen ions are
implanted either to form a buried oxide layer in the silicon wafer topped
by Si in the case of oxygen ion implantation or to separate (exfoliate) a
thin Si layer to bond to another Si wafer with an oxide layer as in the
case of hydrogen ion implantation.

[0005] U.S. Pat. No. 7,176,528 discloses a process that produces an SOG
(semiconductor on glass) structure using an exfoliation technique. The
steps include: (i) exposing a silicon wafer surface to hydrogen ion
implantation to create a bonding surface; (ii) bringing the bonding
surface of the wafer into contact with a glass substrate; (iii) applying
pressure, temperature and voltage to the wafer and the glass substrate to
facilitate bonding therebetween; and (iv) separating the glass substrate
and a thin layer of silicon from the silicon wafer.

[0006] The above approach is susceptible to an undesirable effect under
some circumstance and/or when employed in certain applications. With
reference to FIGS. 1A-1D, a semiconductor wafer 20 is implanted with
ions, e.g., hydrogen ions, through a surface 21, such that the
implantation dose is uniform in terms of density and depth across the
semiconductor wafer 20.

[0007] With reference to FIG. 1A, when a semiconductor material, such as
silicon, is implanted with ions, such as H-ions, damage sites are
created. The layer of damage sites define an exfoliation layer 22. Some
of these damage sites nucleate into platelets with very high aspect
ratios (they have a very large effective diameter and almost no height).
Gas resulting from the implanted ions, such as H2, diffuses into the
platelets to form bubbles of comparably high aspect ratios. The gas
pressure in these bubbles can be extremely high and has been estimated to
be as high as about 10 kbar.

[0008] As illustrated by the bi-directional arrows in FIG. 1B, the
platelets or bubbles grow in effective diameter until they get close
enough to each other that the remaining silicon is too weak to resist the
high pressure of the gas. As there is no preferential point for a
separation front to start, the multiple separating fronts are randomly
created and the multiple cracks propagate through the semiconductor wafer
20.

[0009] Near the edges of the semiconductor wafer 20, a larger share of
implanted hydrogen may escape from the hydrogen rich plane. This is so
because of the proximity of sinks (i.e., the side walls of the wafer 20).
More particularly, during implantation, the ions (e.g., hydrogen protons)
decelerate through the lattice structure of the semiconductor wafer 20
(e.g., silicon) and displace some silicon atoms from their lattice sites,
creating the plane of defects. As the hydrogen ions lose their kinetic
energy, they become atomic hydrogen and define a further, atomic hydrogen
plane. Both the defect plane and the atomic hydrogen plane are not stable
in the silicon lattice at room temperature. Thus, the defects (vacancies)
and the atomic hydrogen move toward one another and form thermally stable
vacancy-hydrogen species. Multiple species collectively create a hydrogen
rich plane. (Upon heating, the silicon lattice cleaves generally along
the hydrogen rich plane.)

[0010] Not all vacancies and hydrogen undergo collapse into
hydrogen-vacancy species. Some atomic hydrogen species diffuse away from
the vacancy plane and eventually leave the silicon wafer 20. Thus, some
of the atomic hydrogen does not contribute to cleavage of the exfoliation
layer 22. Near the edges of the silicon wafer 20, the hydrogen atoms have
an additional path to escape from the lattice. Therefore, the edge areas
of the silicon wafer 20 may be lower in hydrogen concentration. The lower
concentration of hydrogen results in the need for a higher temperature or
longer time to develop enough force to support separation.

[0011] Therefore, during the separation process, a tent-like structure 24
is created with edges that are not separated. At a critical pressure,
fracture of the remaining semiconductor material occurs along relatively
weak planes, such as {111} planes (FIG. 1c) and the separation of the
exfoliation layer 22 from the semiconductor wafer 20 is complete (FIG.
1D). The edges 22A, 22B, however, are out of a major cleavage plane
defined by the damage sites. This non-planar cleavage is not desirable.
Other characteristics of the separation include that the exfoliated layer
22 can be described as having "mesas", where the platelets or bubbles
were, surrounded by "canyons", where the fracture occurred. It is noted
that these mesas and canyons are not accurately shown in FIG. 1D as such
details are beyond the capabilities of reproduction at the illustrated
scale.

[0012] Without limiting the invention to any theory of operation, the
inventors of the instant application believe that the time from the onset
of separation to completed separation is on the order of 10's of
micro-seconds using the techniques described above. In other words, the
random onset and propagation of the separation is on the order of about
3000 meters/sec. Again, without limiting the invention to any theory of
operation, the inventors of the instant application believe that this
rate of separation contributes to the undesirable characteristic of the
cleaved surface of the exfoliation layer 22 described above (FIG. 1D).

[0013] U.S. Pat. No. 6,010,579 describes a technique of uniform ion
implantation into a semiconductor substrate 10 to a uniform depth Z0,
taking the wafer to a temperature below that which would initiate the
onset of separation, and then introducing multiple impulses of energy to
the edge of the substrate 10 in the vicinity of the implant depth Z0 in
order to achieve a "controlled cleave front". U.S. Pat. No. 6,010,579
states that the above approach is an improvement over so-called "random"
cleavage at least as to surface roughness. The instant invention takes a
directed separation approach that is significantly different from the
"controlled cleave front" approach of U.S. Pat. No. 6,010,579 and
different from the "random" cleaving approach.

[0014] The challenges associated with the separation of the exfoliation
layer 22 from the semiconductor wafer 20 discussed above are exacerbated
as the size of the SOI structure increases, and particularly when the
shape of the semiconductor wafer is rectangular. Such rectangular
semiconductor wafers may be used in applications where multiple
semiconductor tiles are coupled to an insulator substrate. Further
details regarding the manufacturing of a tiled SOI structure may be found
in U.S. Application Publication No. 2007/0117354, the entire disclosure
of which is hereby incorporated by reference.

SUMMARY

[0015] For ease of presentation, the following discussion will at times be
in terms of SOI structures. The references to this particular type of SOI
structure are made to facilitate the explanation of the invention and are
not intended to, and should not be interpreted as, limiting the
invention's scope in any way. The SOI abbreviation is used herein to
refer to semiconductor-on-insulator structures in general, including, but
not limited to, silicon-on-insulator structures. Similarly, the SOG
abbreviation is used to refer to semiconductor-on-glass structures in
general, including, but not limited to, silicon-on-glass structures. The
abbreviation SOI encompasses SOG structures.

[0016] In accordance with one or more embodiments of the present
invention, method and apparatus employed in forming a
semiconductor-on-insulator (SOI) structure, provide for: subjecting a
implantation surface of a donor semiconductor wafer to an ion
implantation step to create a weakened slice in cross-section defining an
exfoliation layer of the donor semiconductor wafer; and subjecting the
donor semiconductor wafer to a spatial variation step, either before,
during or after the ion implantation step, such that one or more
parameters of the weakened slice vary spatially across the wafer in at
least one of X- and Y- axial directions.

[0017] The spatial variation step facilitates characteristics of
separation of the exfoliation layer from the donor semiconductor wafer
such that such separation is directionally and/or temporally
controllable.

[0018] The parameters may include one or more of the following, alone or
in combination: (i) densities of nucleation sites resulting from the ion
implantation step; (ii) depths of the weakened slice from the
implantation surface (or the reference plane); (iii) artificially created
damage locations (e.g., blind holes) through the implantation surface at
least to the weakened slice; and (iv) nucleation of defect sites and/or
pressure increases throughout the weakened slice using temperature
gradients.

[0019] The method and apparatus further provide for elevating the donor
semiconductor wafer to a temperature sufficient to initiate separation at
the weakened slice from a point, edge, and/or region of the weakened
slice. The donor semiconductor wafer may be subject to further
temperatures sufficient to continue separation substantially along the
weakened slice directionally as a function of the varying parameter(s).

[0020] Other aspects, features, advantages, etc. will become apparent to
one skilled in the art when the description of the invention herein is
taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] For the purposes of illustrating the various aspects of the
invention, there are shown in the drawings forms that are presently
preferred, it being understood, however, that the invention is not
limited to the precise arrangements and instrumentalities shown.

[0022] FIGS. 1A, 1B, 1C, and 1D are block diagrams illustrating an
exfoliation process in accordance with the prior art;

[0023] FIGS. 2A-2B are block diagrams illustrating an exfoliation process
in accordance with one or more aspects of the present invention;

[0024] FIG. 3A is a top view of a donor semiconductor wafer having a
spatially varying parameter associated with a weakened layer or slice
therein in accordance with one or more aspects of the present invention;

[0025]FIG. 3B is a plot that graphically illustrates the spatially
varying parameter of FIG. 3A;

[0026]FIG. 3C is a plot that graphically illustrates that the spatially
varying parameter of FIG. 3A is the depth of the weakened slice;

[0027] FIGS. 4A, 4B, and 4C are top views of respective donor
semiconductor wafers having further spatially varying parameters in
accordance with one or more further aspects of the present invention;

[0028] FIGS. 5A, 5B, and 5C are simplified diagrams of some ion
implantation apparatus that may be adapted to achieve spatially varying
parameters of the donor semiconductor wafer;

[0029] FIGS. 6A-6B illustrate an ion implantation technique that may be
adapted to achieve a spatially varying density of nucleation sites in the
donor semiconductor wafer;

[0030] FIGS. 7A-7B illustrate an ion implantation technique that may be
adapted to achieve a spatially varying implantation depth in the donor
semiconductor wafer;

[0032] FIGS. 8A-8B illustrate an ion implantation technique that may be
adapted to achieve a spatially varying ion implantation distribution
width in the donor semiconductor wafer;

[0033]FIG. 8c is a graph illustrating a relationship between ion implant
tilt angle and straggle;

[0034] FIGS. 9A-9D illustrate a further ion implantation technique that
may be adapted to achieve a spatially varying ion implantation depth in
the donor semiconductor wafer;

[0035] FIGS. 10A-10D and 11 illustrate a further ion implantation
technique that may be adapted to achieve a spatially varying distribution
of defect sites in the donor semiconductor wafer; and

[0036] FIGS. 12A-12B illustrate a time-temperature profile technique that
may be adapted to achieve a spatially varying parameter profile in the
donor semiconductor wafer.

DETAILED DESCRIPTION

[0037] With reference to the drawings, wherein like numerals indicate like
elements, there is shown in FIGS. 2A-2B an intermediate SOI structure (in
particular, an SOG structure) in accordance with one or more embodiments
of the present invention. The intermediate SOG structure includes an
insulator substrate, such as a glass or glass ceramic substrate 102, and
a donor semiconductor wafer 120. The glass or glass ceramic substrate 102
and the donor semiconductor wafer 120 have been coupled together using
any of the art-recognized processes, such as bonding, fusion, adhesive,
etc.

[0038] Prior to coupling the glass or glass ceramic substrate 102 and the
donor semiconductor wafer 120 together, the donor semiconductor wafer 120
includes an exposed implantation surface 121. The implantation surface
121 of the donor semiconductor wafer 120 is subjected to an ion
implantation step to create a weakened slice 125 in cross-section
defining an exfoliation layer 122. The weakened slice 125 lies
substantially parallel to a reference plane (which could be anywhere, and
thus is not illustrated) defined by X-Y orthogonal axial directions. The
X-axial direction is shown left-to-right in FIG. 2A, while the Y-axial
direction is orthogonal to the X-axial direction into the page (and thus
is not shown).

[0039] The donor semiconductor wafer 120 is subject to a spatial variation
step, either before, during or after the ion implantation step, such that
the characteristics of separation of the exfoliation layer 122 from the
donor semiconductor wafer 120 are directionally and/or temporally
controllable. While not intending to limit the invention to any theory of
operation, it is believed that such directional and/or temporal
controllability may result in improved separation characteristics, such
as smoother exposed surfaces on the exfoliation layer 122 and the donor
semiconductor wafer 120 (post separation). It is also believed that such
directional and/or temporal controllability may result in improved edge
characteristics, e.g., improving the yield of edges of the exposed
surfaces on the exfoliation layer 122 and the donor semiconductor wafer
120 that are in a major cleavage plane defined by the weakened slice 125.

[0040] The directionally and/or temporally controllable characteristics of
separation of the exfoliation layer 122 from the donor semiconductor
wafer 120 may be achieved in a number of ways, such as by varying one or
more parameters spatially across the weakened slice 125 in at least one
of the X- and Y- axial directions. The parameters may include one or more
of the following, alone or in combination: (i) densities of nucleation
sites resulting from the ion implantation step; (ii) depths of the
weakened slice 125 from the implantation surface 121 (or the reference
plane); (iii) artificially created damage locations (e.g., blind holes)
through the implantation surface 121 at least to the weakened slice 125;
and (iv) nucleation of defect sites and/or pressure increases throughout
the weakened slice 125 using temperature gradients.

[0041] As illustrated in FIGS. 2A-2B by the arrow A, the directionally
and/or temporally controllable characteristics of separation of the
exfoliation layer 122 from the donor semiconductor wafer 120 result in a
propagating separation from one point, edge, and/or region to other
points, edges, and/or regions of the weakened slice 125 as a function of
time. This is generally achieved as follows: first, varying the one or
more parameters spatially across the weakened slice 125 as discussed
above, and second, elevating the donor semiconductor wafer 120 to a
temperature sufficient to initiate separation at the weakened slice 125
from such point, edge, and/or region. Thereafter, the donor semiconductor
wafer 120 is elevated to further temperatures sufficient to continue
separation substantially along the weakened slice 125 directionally as a
function of the spatial variation of the parameter(s) across the weakened
slice 125. The varying parameter is preferably established such that a
time-temperature profile of the elevating temperatures is on the order of
seconds, and a propagation of the separation along the weakened slice 125
occurs over at least one second.

[0042] Reference is now made to FIGS. 3A-3C, which illustrate further
details associated with varying the one or more parameters spatially
across the weakened slice 125. FIG. 3A is a top view of the donor
semiconductor wafer 120 viewed through the implantation surface 121. The
variation in shading in the X-axial direction represents the spatial
variation in the parameter (e.g., density of nucleation sites, pressure
within the sites, degree of nucleation, distribution of artificially
created damage sites (holes), implantation depth, etc.). In the
illustrated example, the one or more parameters vary from one edge 130A
in the X-axial direction toward an opposite edge 130B of the donor
semiconductor wafer 120 (and thus the weakened slice 125 thereof), or
vice verse.

[0043] With reference to FIG. 3B, a graph of the separation parameter
illustrates the cross-sectional profile of, for example, the density of
nucleation sites within the weakened slice 125 as a function of the
X-axial direction. Alternatively, or in addition, the separation
parameter may represent one or more of the pressure within the nucleation
sites, the degree of nucleation, the distribution of artificially created
damage sites (holes), etc., each as a function of the X-axial, spatial
metric. With reference to FIG. 3C, a graph of the separation parameter
illustrates the cross-sectional profile of, for example, the depth of the
weakened slice 125 (e.g., corresponding to the ion implantation depth) as
a function of the X-axial direction.

[0044] While not intending to limit the invention to any theory or
theories of operation, it is believed that the propagation of separation
(illustrated by broken arrows) from the edge 130A toward the edge 130B
occurs when the density of nucleation sites is relatively high at edge
130A and reduces to lower densities of nucleation sites at spatial
locations toward the edge 130B. This theory is also believed to hold in
connection with other parameters, such as the gas pressure within the
nucleation sites, the degree of merging nucleation sites prior to
separation, and the distribution of artificially created damage sites
(holes). As to the parameter associated with the depth of the weakened
slice 125, however, it is believed that the propagation of separation
(illustrated by solid arrows) from the edge 130B toward the edge 130A
occurs when a substantially low depth exists along the initial edge 130B
of the weakened slice 125 and comparatively higher depths exist at
successively further distances toward the edge 130A.

[0045] Reference is now made to FIGS. 4A-4C, which illustrate further
details associated with varying the one or more parameters spatially
across the weakened slice 125. The figures show top views of the donor
semiconductor wafer 120 viewed through the implantation surface 121. The
variation in shading in the X-and Y- axial directions represent the
spatial variation in the parameter, again density of nucleation sites,
pressure within the sites, degree of nucleation, distribution of
artificially created damage sites (holes), implantation depth, etc. In
each illustrated case, the parameter is varied spatially in both the X-
and Y- axial directions.

[0046] With specific reference to FIG. 4A, the shading may represent that
the parameter varies spatially starting from two edges 130A, 130D toward
other edges 130B, 130C and varying at successively further distances in
both the X- and Y- axial directions. In keeping with the discussion
above, when considering the parameter of the density of nucleation sites,
if the higher densities initiate at edges 130A, 130D, then it is believed
that the propagation of separation (illustrated by the broken arrow) will
radiate out from the corner of edges 130A, 130D toward the center of the
wafer 120 and toward the other edges 130B, 130C. This theory is also
believed to hold in connection with other parameters, such as the gas
pressure within the nucleation sites, the degree of merging of nucleation
sites prior to separation, and the distribution of artificially created
damage sites (holes). As to the parameter associated with the depth of
the weakened slice 125, however, it is believed that the propagation of
separation (illustrated by the solid arrow) will radiate out from the
corner of edges 130B, 130C toward the center of the wafer 120 and toward
the other edges 130A, 130D when the lower depths low depth initiate along
the edge 130B, 130C.

[0047] With specific reference to FIGS. 4B and 4C, the shading may
represent that the parameter varies spatially starting from all edges 130
and varying toward the center of the donor semiconductor wafer 120, or
vice verse.

[0048] Further details will now be provided with reference to the
particular parameter of spatially varying the densities of nucleation
sites resulting from ion implantation across the weakened slice 125 in
one or both of the X- and Y- axial directions. No matter what technique
is employed to achieve such spatial variation, it is preferred that a
maximum density of nucleation sites exists at one or more edges, points,
or regions of the weakened slice 125 of about 5×105
sites/cm2 and a minimum density of nucleation sites exists spaced
away therefrom in the weakened slice 125 of about 5×104
sites/cm2. Looking at the variation in another way, a difference
between the maximum density of nucleation sites and the minimum density
of nucleation sites may be between about 10 fold.

[0049] In accordance with one or more aspects of the present invention,
the density of nucleation sites within the weakened slice 125 may be
varied spatially by varying the dose of the ion implantation step. By way
of background, the weakened slice 125 (and thus the exfoliation layer
122) is created by subjecting the implantation surface 121 to one or more
ion implantation steps. Although there are numerous ion implantation
techniques, machines, etc. that may be utilized in this regard, one
suitable method dictates that the implantation surface 121 of the donor
semiconductor wafer 120 may be subject to a hydrogen ion implantation
step to at least initiate the creation of the exfoliation layer 122 in
the donor semiconductor wafer 120.

[0050] With reference to FIG. 5A, a simplified schematic of an Axcelis
NV-10 type batch implanter is illustrated, which may be modified for use
in spatially varying the density of nucleation sites within the weakened
slice 125 by varying the dose of implanted ions.

[0051] Multiple donor semiconductor wafers 120, in this case rectangular
tiles, may be distributed azimuthally at a fixed radius on a platen 200
relative to the incident ion beam 202 (directed into the page). Rotation
of the platen 200 provides a pseudo-X-scan (dX/dt) while mechanical
translation of the entire platen 200 provides the Y-scan (dY/dt). The
term pseudo-X-scan is used because for small radius platens 200, the
X-scan is somewhat more curved as compared to larger radius platens 200,
and thus, perfectly straight scans are not obtained on such rotating
platens 200. Modulating the X-scan speed and/or the Y-scan speed will
result in spatial variation in the dose. Increasing the Y-scan speed as
the ion beam 202 travels radially toward the center of the platen 200 has
been used in the past to ensure a uniform dose. Indeed, as the
conventional thinking in the art is to achieve a spatially uniform dose,
and as the angular speed relative to the donor semiconductor wafers 120
decreases closer to the center of the platen 200, the Y-scan speed must
correspondingly increase. In accordance with the invention, however, a
spatially varying dose may be achieved by not adhering to the
conventional scan protocol, resulting in the patterns of, for example,
FIGS. 3A and 4A. For example, leaving the Y-scan speed uniform as the ion
beam 202 travels radially toward the center of the platen 200.
Alternatively, one could decrease the Y-scan speed as the ion beam 202
travels radially toward the center of the platen 200. Those skilled in
the art will recognize other possibilities from the disclosure herein. An
alternative approach is to vary the beam energy as a function of the scan
rates and positions. These changes may be effected through modification
to the control algorithm of the implanter in software, an electronic
interface between the controlling software and the end station drive, or
other mechanical modification.

[0052] With reference to FIG. 5B, a simplified schematic of a
single-substrate X-Y implanter is illustrated, which also may be modified
for use in spatially varying the density of nucleation sites within the
weakened slice 125 by varying the dose of implanted ions. In this case,
the electronic beam 202 is scanned much faster than the mechanical
substrate scan (of FIG. 5A). Again, the conventional thinking in the art
is to achieve a spatially uniform dose, and thus the X and Y scanning
rates and beam energy are set such that the uniform dose is achieved.
Again, spatially varying dosages may be achieved by not adhering to the
conventional scanning protocol. Significant spatial variation in implant
dosage may be achieved through numerous combinations of variable X and Y
scanning rates and/or beam energy. One-dimensional or two-dimensional
gradients may be produced, either vertical or horizontal, through such
variation resulting in the patterns of, for example, FIGS. 3A, 4A, 4B and
4C.

[0053] With reference to FIG. 5c, a simplified schematic of an implanter
is illustrated in accordance with ion shower techniques. A ribbon beam
204 arises from an extended ion source. In accordance with conventional
techniques, a single uniform speed scan (in proportion with a uniform
beam energy in the orthogonal direction) can achieve conventional ideals,
i.e., a spatially uniform dose. In accordance with various aspects of the
invention, however, a one-dimensional gradient (e.g., that of FIG. 3A
rotated 90 degrees) may be produced through variation in the mechanical
scanning rate of the donor semiconductor wafers 120 through the ribbon
beam 204. Twisting the donor semiconductor wafers 120 by some angle
relative to the ribbon beam 204 in combination with variation in the
mechanical scanning rate may produce spatial variation in the dose in a
manner similar to that of FIG. 4A. Alternatively or additionally, a
spatially varying beam current along the beam source would provide an
orthogonal gradient to the scan direction, providing additional degrees
of freedom to produce the subject spatially varying dosages.

[0054] Irrespective of the particular implantation technique employed to
achieve the variation in dose, and irrespective of the location of the
highest dose (e.g., along one or more initial edges, an initial point, or
an initial region), the substantially highest dose is within some
desirable range in units of atoms/cm2 and the lowest dose further
therefrom in at least one of the X- and Y- axial directions is within
some other desirable range in units of atoms/cm2. A difference
between the maximum dose and the minimum dose may be between about
10-30%, with a maximum variation of about a factor of three. In some
applications, a difference of at least about 20% has been found to be
important.

[0055] In accordance with one or more further aspects of the present
invention, the density of nucleation sites within the weakened slice 125
may be varied spatially by implanting a first species of ions in a
substantially uniform manner to establish the weakened slice 125 with a
substantially uniform distribution. Thereafter, the donor semiconductor
wafer 120 may be implanted with a second species of ions in a
substantially non-uniform manner. The non-uniform implantation is
established such that the second species of ions causes migration of
atoms to the weakened slice 125 resulting in the spatially varying
densities of nucleation sites across the weakened slice 125.

[0056] By way of example, the first species of ions may be hydrogen ions
and the second species of ions may be helium ions.

[0057] The non-uniform implantation may take place using the techniques
described above, described later in this description, or gleaned from
other sources. For example, the dose of the second species of ions may be
spatially varied. The variation in the dose of the second species of ions
(such as He ions) will cause a subsequent non-uniform migration of the
second species to the location of the first species, thereby establishing
a non-uniform density of nucleation sites. This variation will probably
also vary the pressure in the platelets, which could also be beneficial.

[0058] Alternatively, the non-uniform implantation of the second species
of ions may include implanting the second species of ions to varying
depths spatially across the donor semiconductor wafer 120. Any of the
known techniques for implanting ions to uniform depths may be modified by
those skilled in the art in accordance with the teaching herein to
achieve non-uniform depth profiles. By way of background, it is known
that He ions can be implanted deeper than H, for example, as much as two
times deeper or more. As the wafer temperature increases, much of the He
ions will migrate to the site of shallower H ion implants and will
provide the gas pressure for later separation. In accordance with the
instant aspect of the invention, the damage caused by more deeply buried
He is located at a depth in the donor semiconductor wafer 120 far from
the shallower H ion implant and fewer of such He ions will arrive there
in a given time. The opposite is true for the less deeply implanted He
ions, thereby resulting in a spatially varying density of nucleation
sites across the weakened slice 125.

[0059] While, theoretically, the spatially varied density of nucleation
sites may be achieved irrespective of the order of the first and second
species of ions (e.g., He implanted first or H implanted first), the
order of the multiple ion implantation steps may also contribute to the
desired result. Indeed, the order of implantation, depending on ion
species, may have an overall effect on the density even as the density
also varies spatially. While counterintuitive and surprising to many
skilled artisans, it has been found that H implanted first creates more
nucleation sites. For a given dose, He is recognized by skilled artisans
to produce about ten times the damage as H ions. It should be noted,
however, that the damage produced by the He ions (a vacancy and
interstitial semiconductor atom, or Frankel pair) self anneals rapidly
even at room temperature. Thus, much, but not all, of the He damage is
repaired. H ions, on the other hand, bond with semiconductor atoms, such
as Si atoms (forming an Si--H bond), and stabilize the damage that is
created. If H is present before the He is implanted, more nucleation
sites are created.

[0060] Reference is now made to FIGS. 6A-6B, where a further example is
illustrated that may be suitable for achieving the spatial variation in
the density of nucleation sites. In this example, as illustrated in FIG.
6A, the spatial variation in the density of nucleation sites is achieved
by adjusting the beam angle of the ion beam during the ion implantation
step. Although the beam angle may be adjusted in a number of ways, one
such approach is to tilt the donor semiconductor wafer 120 with respect
to the ion beam (e.g., a dot beam 202) as illustrated in FIG. 6A. The
donor semiconductor wafer 120 has a width (left-to-right as shown on the
page), a depth (into the page) and a height (top-to-bottom as shown on
the page). The width and depth may define the X- and Y- axial directions,
and the height may define a longitudinal axis, Lo, normal to the
implantation surface 121. The donor semiconductor wafer 120 is tilted
such that the longitudinal axis Lo thereof is at an angle (I) with
respect to a directional axis of an ion implantation beam (shown as a
solid arrow) during the ion implantation step. The angle Φ may be
between about 1 to 45 degrees.

[0061] Under a tilt condition, as the beam source scans from location A to
location B, the width W of the beam 202 varies at the implantation
surface 121 of the donor semiconductor wafer 120 from width Wa to width
Wb, or vice verse. The variation in width W contributes to a variation in
the densities of nucleation sites resulting from the ion implantation in
the scanning directions (which may be set up to vary along at least one
of the X- and Y- axial directions).

[0062] The implant beam 202 may include hydrogen ions, which have the same
(positive) electrical charge. As particles with the same charge repel
each other, the beam 202 is wider at a longer distance from ion source
(position A), and narrower at a shorter distance from ion source
(position B). The more focused (lower width Wb) ion beam at position B
heats the local area of the donor semiconductor wafer 120 to a higher
degree than the less focused (higher width Wa) ion beam at position A.
Under higher temperature, more hydrogen ions diffuse out from such local
area, and a lower share of hydrogen ions remain as compared to other
areas. As illustrated in FIG. 6B, this results in laterally non-uniform
distribution of hydrogen (and thus the density of nucleation sites) in
the weakened slice 125 of the donor semiconductor wafer 120.

[0063] Similar spatial variation in the density of nucleation sites may be
achieved by adjusting the angle of the beam source or incorporating some
of the known mechanisms for adjusting the collimation of the ion beam
202.

[0064] A further technique that may be suitable for achieving the spatial
variation in the density of nucleation sites is to employ a two-stage ion
implantation step. A first ion implantation is performed to implant ions
that have the effect of attracting a second species of ions. Thereafter,
the second species of ions are implanted. The first species of ions are
implanted in a spatially non-uniform manner, using any of the suitable
techniques described above or later herein. Thus, when the second species
of ions are implanted, and migrate to the first species, the resultant
weakened slice 125 exhibits a non-uniform density of nucleation sites.

[0065] For example, the first ion species may be based on the material of
the donor semiconductor wafer 120, such as using silicon ions for
implantation in a silicon donor semiconductor wafer 120. Such Si ions may
have the property of trapping a second species of ions, such as hydrogen
ions. As noted above, H ions bond with some semiconductor atoms, such as
Si atoms, forming an Si--H bond. As an example, silicon-into-silicon
implantation may be performed at doses and energies known in the art,
such as is described in U.S. Pat. No. 7,148,124, the entire disclosure of
which is incorporated by reference. Unlike the prior art, however, a
spatial density distribution of the trapping ion specie (in this case Si)
is non-uniform (e.g., highest at one edge and lowest on an opposite edge
of the donor semiconductor wafer 120, or other variations discussed
herein). Next, a second species of ions, such as hydrogen, is implanted,
which may be a uniform distribution. The amount of hydrogen remaining in
the weakened slice 125 of the donor semiconductor wafer 120 will depend
on two factors: (1) the concentrating distribution of sites that are able
to trap the second species, hydrogen, and (2) the available hydrogen (the
hydrogen implanted and remaining from the implant dose).

[0066] It is noted that the non-uniform spatial distribution of the
species may be reversed to achieve a similar result. For example, the
first species may implanted uniformly, followed by a non-uniform
implantation of the second species. Alternatively, both implants may be
spatially non-uniform. The non-uniform distribution of the second species
(e.g., hydrogen) within the weakened slice 125 results in a point, edge
or region of highest concentration of hydrogen, which in turn is location
of the lowest temperature for initiating cleavage.

[0067] Again, with reference to FIGS. 2A-2B, the arrow A illustrates the
directionally and/or temporally controllable characteristics of
separation of the exfoliation layer 122 from the donor semiconductor
wafer 120, where a propagating separation from one a point, edge, and/or
region to other points, edges, and/or regions of the weakened slice 125
is achieved as a function of time. In the context of spatial variation of
the density of nucleation sites, the donor semiconductor wafer 120 is
elevated to a temperature sufficient to initiate separation at the
weakened slice 125 from a point, edge, and/or region of highest density.
It has been found that high hydrogen concentrations in silicon allows
separation at temperatures as low as 350° C. or lower, while
silicon with lower concentrations of hydrogen separates at higher
temperatures, such as 450° C. or more. The donor semiconductor
wafer 120 is elevated to further temperatures sufficient to continue
separation substantially along the weakened slice 125 directionally as a
function of the spatial variation of the density across the weakened
slice 125.

[0068] Further details will now be provided with reference to the
particular parameter of spatially varying the depth of the weakened slice
125 resulting from ion implantation in one or both of the X- and Y- axial
directions. No matter what technique is employed to achieve such spatial
variation, it is preferred that a substantially low depth is between
about 200-380 nm and a highest depth is between about 400-425 nm. Looking
at the variation in another way, a difference between the maximum and
minimum depths may be between about 5-200%.

[0069] In accordance with one or more aspects of the present invention,
the depth of the weakened slice 125 may be varied spatially by adjusting
beam angle of the ion beam during the ion implantation step. Indeed, the
process discussed with respect to FIGS. 6A-6B may have applicability to
adjusting the depth of the weakened slice 125. (It is noted that the
mechanism of varying temperature as a function of beam width is not
believed to be the reason that variations in the depth of the weakened
slice 125 are achieved.)

[0070] With reference to FIGS. 6A, and 7A-7B, the spatial variation in the
depth of the weakened slice 125 may be achieved by varying at least one
of: (1) the angle Φ of tilt (shown and described with reference to
FIG. 6A); and (2) a twist of the donor semiconductor wafer 120 about the
longitudinal axis Lo thereof with respect to the directional axis of the
ion implantation beam 202. Adjustments in the tilt and/or twist are made
to adjust a degree of channeling through the lattice structure of the
donor semiconductor wafer 120, where such channels tend to align and
misalign with the ion beam 202 as the ion beam 202 scans across the
implantation surface 121. As the degree of channeling varies spatially,
so does the depth of the weakened slice 125.

[0071] The angle Φ may be between about 1-10 deg degrees and the angle
of twist may be between about 1-45 degrees.

[0072] As inferred above, and with further reference to FIGS. 7C and 7D,
implant depth gets smaller as tilt gets bigger. For relatively small
angles (e.g., 0-10 deg), the relationship between implant depth and tilt
is dominated by channeling. For relatively larger angles, the cosine
effect dominates. In other words, the resultant exfoliation film
thickness is essentially proportional to the cosine of the implant angle.

[0073] Alternatively or additionally, the spatial variation step may
include varying an energy level of the ion beam 202 such that as the ion
beam 202 scans across the implantation surface 121 of the donor
semiconductor wafer 120, depths of the weakened slice 125 from the
implantation surface 121 vary spatially across the donor semiconductor
wafer 120.

[0074] As illustrated in FIG. 7B, the above techniques results in a
laterally non-uniform depth of the weakened slice (or implant depth) of
the donor semiconductor wafer 120.

[0075] In connection with adjusting the tilt of the donor semiconductor
wafer 202, a further parameter that may be exploited to achieve spatial
variations is the width of the ion deposition distribution (or straggle).
As illustrated in FIG. 8A, the width of the ion distribution through the
weakened slice 125 (top-to-bottom) varies as a function of the angle of
the tilt of the donor semiconductor wafer 120 (or more generally the beam
angle). Thus, by varying the tilt angle, a spatially varying distribution
width may be achieved in the weakened slice 125 (as illustrated in FIG.
8B). While not intending to be limited by any theory of operation, it is
believed that the portions of the weakened slice 125 having narrower
distribution widths will separate at lower temperatures than the portions
of the weakened slice 125 having wider distribution widths. Thus, it is
believed that directionally and/or temporally controllable
characteristics of separation of the exfoliation layer 122 from the donor
semiconductor wafer 120, where a propagating separation from one a point,
edge, and/or region to other points, edges, and/or regions of the
weakened slice 125 may be achieved as a function of time and temperature.

[0076] With reference to FIG. 8c, additional data regarding the effect of
the tilt on the straggle, which again has an impact on the width of the
implant profile. The dose used in both implants illustrated in FIG. 8c
are the same. Although the peak H concentration is different, both
implants exfoliate. Thus, the difference between a tilt variation of
+/-0.1 deg and +/-3 deg is significant for straggle.

[0077] With reference to FIGS. 9A-9D, another technique for spatially
varying the depth of the weakened slice 125 includes subjecting the donor
semiconductor wafer 120 to a post implantation material removal process
such that the depths of the weakened slice 125 from the implantation
surface 121 vary spatially across the donor semiconductor wafer 120. As
illustrated in FIG. 9A, the donor semiconductor wafer 120 may be subject
to some deterministic polishing process or plasma-assisted chemical
etching (PACE). These techniques permit local control of the amount of
material removed by the polishing step. Other methods, including Reactive
Ion Etching (RIE), Chemical Mechanical Polishing (CMP), and wet chemical
etching may also have non-uniform material removal across the exposed
surface which is regular and reproducible. One or more of these or other
techniques may be used to introduce slight variation in the depth of the
weakened slice 125 from the implantation surface 121, such as any of
those illustrated in FIGS. 3A, 4A, 4B, 4C, and others. The ion
implantation step prior to material removal may be spatially uniform or
non-uniform.

[0078] With reference to FIGS. 9B and 9C, the spatial variation step may
include using a mask 220A or 220B on the implantation surface 121 of the
donor semiconductor wafer 120 in a spatially non-uniform manner such that
penetration of the ions is impeded to varying degrees as the ion beam 202
scans across the implantation surface 121. The masking film 220 may
include silicon dioxide, organic polymers such as photoresist, and
others. Possible deposition techniques include plasma-enhanced chemical
vapor deposition (PECVD), spin coating, Polydimethylsiloxane (PDMS)
stamping, etc. The masking film 220 thickness may be less than or
comparable to the intended depth of the weakened slice 125. As the depth
to which ions are implanted is determined by the energy of the incident
ions, the impeding action of the mask 220 will translate into spatial
modulation in primarily the depth of the implanted species in the donor
semiconductor wafer 120. Depending on the characteristics of the
deposited mask 220, the desired characteristic may be achieved by adding
length to the ion path, scattering the ions to alter the degree of
channeling, or other phenomena.

[0079] As illustrated in FIG. 9D (which illustrates lower depths on all
edges of the weakened slice 125 and higher depths toward the center
thereof), after or during bonding to the substrate 102, the donor
semiconductor wafer 120 is elevated to a temperature sufficient to
initiate separation at the weakened slice 125 from a point, edge, and/or
region of lowest depth. The donor semiconductor wafer 120 is elevated to
further temperatures sufficient to continue separation substantially
along the weakened slice 125 directionally as a function of the spatial
variation of the depth from lowest depth to highest depth.

[0080] With reference to FIGS. 10A-10D and 11, the spatial variation step
may include boring one or more blind holes 230 through the implantation
surface 121 at least to the weakened slice 125, and preferably through
the weakened slice 125 (FIG. 10B). While not intending to limit the
invention to any theory of operation, it is believed that during or after
bonding to the substrate 102 (FIG. 10c), elevating the donor
semiconductor wafer 120 to higher temperature will initiate separation at
the blind hole 230 (FIG. 10D) prior to separation at locations without
such hole. As illustrated in FIG. 11, boring an array of blind holes 230
through the implantation surface 121 may create a non-uniform spatial
distribution of such holes. Thus, elevating the donor semiconductor wafer
120 to temperatures sufficient to initiate and continue separation
substantially along the weakened slice 125 may be achieved directionally
as a function of the distribution of the array of blind holes 230, from
highest to lowest concentration.

[0081] With reference to FIGS. 12A-12B, the spatial variation step may
include subjecting the donor semiconductor wafer 120 to a non-uniform
time-temperature profile such that the nucleation site density or
pressure at respective spatial locations throughout the weakened slice
125 vary spatially across the donor semiconductor wafer 120. For example,
the illustrated temperature gradient in FIG. 12A applies a higher
temperature to the left side of the wafer 120 as compared to the right
side. This temperature gradient may be applied either before bonding or
in-situ during bonding to the substrate 102. Over time, if the process
time is kept below the separation threshold for the given process
temperature, at least one of the nucleation of defect sites and the gas
pressure therein increases throughout the weakened slice 125 in varying
degrees, spatially across the wafer 120 as a function of the temperature
gradient (see FIG. 12B). The separation threshold time for a given
process temperature is expected to follow an Arrhenius relationship,
where the separation threshold time is exponentially proportional to the
inverse of the process temperature. The parameter of interest is the
ratio of the process time to the separation threshold time at the process
temperature. Any of the aforementioned spatially varying parameter
profiles discussed herein or otherwise desirable may be achieved by
adjusting the process time-separation time ratio profile. Then, the donor
semiconductor wafer 120 is elevated to a temperature sufficient to
initiate separation at the weakened slice 125 from a point, edge, and/or
region of maximum process time-separation time ratio. In the illustrated
example, the maximum process time-separation time ratio is on the left
side of the wafer 120. The donor semiconductor wafer 120 is then elevated
to further temperatures sufficient to continue separation substantially
along the weakened slice 125 directionally as a function of the varying
time-temperature profile, from maximum process time-separation time
ratio(s) to minimum process time-separation time ratio(s). Depending on
material characteristics and other factors, including ion species, dose,
and implant depth, the substantially high process time-separation time
ratio is between about 0.9 and 0.5 and a lowest process time-separation
time ratio is between about 0 and 0.5.

[0082] Various mechanisms may be used pre-bonding or in-situ bonding to
achieve the spatially varying time-temperature profile. For example, one
or more spatially non-uniform conductive, convective, or radiating
heating techniques (hotplate, laser irradiation, visible/infrared lamp,
or other) may be employed to heat the donor semiconductor wafer 120.
Controlled time/temperature gradients may be achieved by direct or
indirect thermal contact (conduction) to achieve any of the desirable
profiles. An addressable, two-dimensional array of hotplate elements may
be used to achieve different profiles based on computer control or
programming. Localized infrared radiation, employing, for example, a lamp
as used in rapid thermal annealing (radiation) may be employed, and/or
visible or near-infrared laser radiation may be used to provide localized
and spatially non-uniform heating (radiation). Alternatively, application
of a uniform or non-uniform thermal profile through any means and
application of a spatially non-uniform cooling mechanism, such as direct
contact (conductive), or gas or fluid flow jets (conductive/convective),
may be employed to achieve the desired time-temperature gradient.

[0083] Again, these heating/cooling techniques may be used pre-bonding or
in-situ. In connection with in-situ bonding techniques, the bonding
apparatus described in, for example, U.S. patent application Ser. No.
11/417,445, entitled HIGH TEMPERATURE ANODIC BONDING APPARATUS, the
entire disclosure of which is hereby incorporated by reference, may be
adapted for use in accordance with the present invention. Management of
thermal radiation loss in the bonding apparatus may be controlled, and
thus exploited to achieve the time-temperature gradient, through the
incorporation of infrared reflecting elements around the perimeter of the
bonding apparatus to minimize radiation loss and maximize edge
temperature. Conversely, management of thermal radiation loss in the
bonding apparatus may be controlled through the incorporation of cooled
infrared absorbers to maximize radiation loss and minimize edge
temperature. Many variations on the above themes may be used to achieve
the desired time-temperature gradient.

[0084] Although the invention herein has been described with reference to
particular embodiments, it is to be understood that these embodiments are
merely illustrative of the principles and applications of the present
invention. It is therefore to be understood that numerous modifications
may be made to the illustrative embodiments and that other arrangements
may be devised without departing from the spirit and scope of the present
invention as defined by the appended claims.